Gas-impervious electrical feedthrough for use between two zones of differing pressures



Nov. 23, 1965 N. vAssos 3,219,753

GAS-IMPERVIOUS ELECTRICAL FEEDTHROUGH FOR USE BETWEEN TWO ZONES 0FDIFFERING PRESSURES Filed May 15, 1965 5 Sheets-Sheet l METALLIC SEALINGMEANS E fl /52722 7 5707/ 754 a 021 533 777%) MM %w@ 5 Sheets-Sheet 2Nov. 23, 1 N. VASSOS GAS-IMPERVIOUS ELECTRICAL FEEDTHRCUGH FOR USEBETWEEN TWO ZONES OF DIFFERING PRESSURES Filed May 15, 1963 flzbXaiQ: asag NOV. 23, 1965 VASSQS 3,219,753

GAS-IMPERVIOUS ELECTRICAL FEEDTHROUGH FOR USE BETWEEN TWO ZONES OFDIFFERING PRESSURES Filed May 15, 1963 3 Sheets-Sheet 3 f fl/ afg m. I U

United States Patent GAS-IMPERVIOUS ELECTRICAL FEEDTHROUGH FOR USEBETWEEN TWO ZONES OF DIFFERING PRESSURES Nicholas Vassos, Urbana, Ill.,assignor to University of Illinois Foundation, a non-profit corporationof Illinois Filed May 15, 1963, Ser. No. 280,568 7 Claims. (Cl. 174152)The present invention relates generally to electrical feedthroughs whichextend through a wall to electrically connect a pair of terminals onrespective opposite sides of the wall, and more particularly tofeedthroughs which electrically connect a first terminal in a zone ofvery low pressure (e.g., 1X10 torr or lower) and a second terminal in azone of relatively high pressure (e.g., atmospheric pressure).

Such pressure differentials exist between the interior and exterior of alow pressure testing system vessel. In such a system the extremely lowinterior pressures are obtained after the entire system, including theelectrical feedthroughs, are assembled in place. Typically, the interiorof the vessel is initially partially evacuated by conventional roughingpumps; and, to assure attainment and maintenance of the desired lowpressures, the entire vessel is subjected to a bake-out which causesgaseous molecules in the walls of the vessel to be emitted into eitherthe atmosphere or the interior of the vessel from where these gaseousmolecules may be removed by conventional pumping means (e.g., adiffusion pump).

It is important that electrical feedthroughs for use in such lowpressure systems constitute a sealed assembly of the various componentsto prevent entry of gases from the high pressure zone to the lowpressure zone between which the feedthrough extends. It is alsoimportant that the feedthrough be capable of withstanding theaforementioned bake-out without breaking the seal among the variouscomponents.

In accordance with the present invention, an electrical feedthrough isprovided, for extension between a zone of very low pressure and a zoneof relatively high pressure, which feedthrough includes a sealingarrangement among the components which prevents leakage of gas from thehigh pressure zone through the feedthrough components to the lowpressure zone, and which will withstand the heat of the bake-out withoutinterrupting the seal among the components.

Other features and advantages are inherent in the structure claimed anddisclosed or will become apparent to those skilled in the art from thefollowing detailed description in conjunction with the accompanyingdrawings wherein:

FIGURE 1 is a perspective view of an embodiment of a feedthroughconstructed in accordance with the present invention;

FIGURE 2 is a vertical sectional view of the feedthrough of FIGURE 1;

FIGURE 3 is a vertical sectional view illustrating the components of theembodiment of FIGURES 1 and 2 prior to the assembly of the components;

FIGURE 4 is a perspective view illustrating an assembly includingseveral feedthroughs of the type illustrated in FIGURE 1;

FIGURE 5 is a vertical sectional view illustrating in detail theattachment of a feedthrough to the assembly shown in FIGURE 4;

FIGURE 6 is a perspective view, partially broken away, and partially insection, illustrating another embodiment of a feedthrough constructed inaccordance with the present invention;

FIGURE 7 is a vertical sectional view of the feedthrough of FIGURE 6;

FIGURE 8 is a vertical sectional view illustrating the components of thefeedthrough of FIGURES 6 and 7 prior to assembly of the components;

FIGURE 9 is a vertical sectional view illustrating the attachment of thefeedthrough of FIGURE 6 to an assembly constituting a portion of asystem with which the feedthrough is intended to be used;

FIGURE 10 is a vertical sectional view illustrating components of afurther embodiment of a feedthrough, in accordance with the presentinvention, prior to the assembly of the components;

FIGURE 11 is a fragmentary vertical sectional view illustrating a stepin assembling a component of the embodiment of FIGURE 10 in a tubulararrangement;

FIGURE 12 is a fragmentary vertical sectional view, similar to FIGURE11, with the component in an assembled condition with respect to thetubular arrangement;

FIGURE 13 is a fragmentary vertical sectional view illustrating thecomponents shown in FIGURE 10 in an assembled condition;

FIGURE 14 is a vertical sectional view illustrating components of stillanother embodiment of a feedthrough, in accordance with the presentinvention, with the components in an unassembled condition;

FIGURE 15 is a vertical sectional view of the embodiment of FIGURE 14with the components in an assembled condition;

FIGURE 16 is a fragmentary perspective view illustrating anotherembodiment of an assembly for feedthroughs of the type illustrated inFIGURE 1; and

FIGURE 17 is a vertical sectional View of the assembly of FIGURE 16.

Referring initially to FIGURES 1 through 3, there is illustrated anembodiment, indicated generally at 20, of an electrical feedthroughconstructed in accordance with the present invention. Embodiment 20includes a first member in the form of a wire or rod 21 composed ofmolybdenum, a metal having good electricity-conducting characteristics;a mounting cup or member, indicated generally at 22, also composed ofmolybdenum; and a glass sealing means 27 constituting a seal andinsulator between cup 22 and rod 21 and maintaining them in the relativepositions illustrated in FIGURES 1 and 2.

Rod 21 and cup 22 are initially supported, in an unassembled condition,on a graphite jig 30 in the relative positions illustrated in FIGURE 3.A tubular glass slug 29, having an opening 31 for receiving rod 21, ispositioned atop a rearward face 24 of cup 22. Rod 21 extends through anopening 26 in a dimple 25 extending in a forward direction axially ofrod 21. Also extending forwardly from cup 24, in the same direction asdimple 25, is a peripheral cup flange 23.

T o obtain a tenacious bond between glass seal 27 and the surfaces ofrod 21 and cup 22, the latter two are, before assembly of thecomponents, subjected to an oxidizing treatment, or are coated withchrome which in turn is subjected to an oxidizing treatment. As aresult, the molybdenum wire 21 and cup 22 are coated with a tenaciousoxide which is readily bondable to the glass of seal 27. One typicaloxidizing treatment comprises heating the molybdenum wire and cup with atorch in air. Another treatment comprises coating the molybdenum rod andcup with chromium and heating them in a wet hydrogen atmosphere. Thisproduces a very tenacious chrome oxide on the surface of the molybdenumrod and cup.

After the unassembled components have been arranged in the configurationillustrated in FIGURE 3, the arrangement is heated to melt glass slug 29which flows downwardly into the recess formed by dimple 25 and throughopening 26 in dimple 25 into the recess 32 in graphite jig 30. Thearrangement is then cooled and removed from the jig as a completedfeedthrough of assembled components illustrated in FIGURES l and 2. Inthe assembled condition, rod 21 and cup 22 are bonded together, in asealed gas-tight arrangement, by the glass means 27 having a portion 28extending through opening 26 in cup dimple 25.

To assure the desired type of seal between the components 21, 22, and27, and to assure that the seal is maintained during the subsequentbake-out to which the feedthrough is subjected (as a part of theaforementioned system), it is important that seal 27 be composed of aglass having thermal expansion characteristics substantially identicalto those of cup 22 and rod 21. The latter two components are composed ofmolybdenum, and glass compositions having the same thermal expansioncharacteristics as molybdenum, are alumino silicate glasses identifiedas Codes 1720 and 1723 by Corning Glass Works, Corning, New York. Theseglasses are described in a bulletin prepared by the Industrial BulbSales Department of Corning Glass Works, Corning, New York, issuedFebruary 15, 1961, and entitled, Product Information, Calcium AluminoSilicate Glass, Glass Codes 1720 and 1723.

Because the glass of seal 27 has the same thermal expansion andcontraction characteristics as the molybdenum from which rod 21 and cup22 are composed, the glass seal 27 and the molybdenum rod and cup 21,22, respectively, will remain in tight, sealing engagement, during allheat-ups and cool-downs to which the feedthrough is subjected, whetherthe heat-ups and cooldowns occur during the assembly of the componentsof the feedthrough or during the subsequent bake-outs to which thefeedthrough may be subjected.

Another important property for the components of feedthrough 20 is animperviousness to gas, so as to prevent gas leakage through theindividual components of the feedthrough when the latter extends betweenzones of vastly different pressures. With respect to cup 22,cross-rolling the molybdenum sheet material from which cup 22 isfabricated increases the density of the molybdenum cup in alldirections, and the problem of gasleakage between grains in themicrostructure of the molybdenum cup material is essentially eliminated.

Because molybdenum rod 21 is subjected to deformation in a longitudinaldirection, during the drawing of the rod, gas leakage radially throughthe rod and outwardly through the side surfaces thereof is not aproblem. However, the microcrystalline grains in the microstructure ofrod 21 are elongated in the direction of drawing, and leakage throughthe tip or free end of rod 21 may be a problem. To offset this problem,the tip of rod 21 which is exposed to the relatively high pressure zoneis coated by fusing thereto a brazement 39 (e.g., composed ofcopper-gold brazing alloy), as illustrated in FIGURE 2. This preventsthe leakage of gas from a relatively high pressure zone through the tipof rod 21.

Seal 27 has good gas-impervious properties when composed of a glass ofthe type described above (Corning Codes 1720 and 1723).

Referring to FIGURES 4 and 5, there is illustrated an assembly 40including a plurality of feedthroughs 20. Assembly 40 includes a ring34, composed of a material such as stainless steel, and a flange orplate 35 mounted and attached to the top of ring 34, as by brazing.Plate 35 is preferably composed of copper, for reasons to besubsequently described, and includes a plurality of openings 36 eachdefined by an upstanding rim 37. Rim 37 is dimensioned to be slidablyreceived within flange 23 of cup 22, in the manner illustrated in FIGUREand rim 37 thus functions to maintain cup 22 in a centered position,preparatory to attaching feedthrough to assembly 40 by brazing cupflange 23 to assembly plate 35, e.g., at 38. Brazing 33 may be composedof copper-gold.

It is desirable that plate be composed of copper because the latter isrelatively resilient, and, during brazing of cup flange 23 to assemblyplate 35, expansion and contraction of the two can be accommodated bythe relatively resilient plate rim 37 which will yield and then springback in accordance with said expansion and contraction. This also helpsto assure a tight seal among the components of assembly 40.

Referring now to FIGURES 6 and 7, there is illustrated anotherembodiment, indicated generally at 50, of a feedthrough constructed inaccordance with the present invention. Feedthrough includes a rod orwire 51, composed of molybdenum, a mounting tube 52 composed of an alloysuch as Kova-r (registered trademark of Westinghouse ElectricCorporation), having thermal expansion and contraction characteristicssubstantially identical to those of molybdenum rod 51; and glass means54- constituting a seal holding tube 52 and rod 51 together in agas-impervious, sealed, electricity-conducting feedthrough.

The .feedthrough components 51, 52, 54 are initially assembled byarranging them in the configuration shown in FIGURE 8. An opening 40 ina graphite jig 56 supports rod 51, and a jig upper surface 41 supports aflange 53 of tube 52, to mount the rod and the tube in upright coaxialpositions. A glass slug 57 is supported atop an inwardly crimpedterminal portion 43 on tube 52, and slug 57 has an opening 53 forreceiving rod 51. The entire arrangement is subjected to heat, therebymelting the glass and causing it to flow; so that upon cooling andsolidification of the glass the components are arranged in theconfiguration shown in FIGURES 6 and 7.

In this configuration all of glass seal 54 encloses and insulates anintermediate portion 65 of rod 51, and substantially all of seal 54 isenclosed within tube 52. In very high voltage systems, it is desirableto remove a portion 66 of tube 52 adjacent the uninsulated part 67 ofrod 51 which projects upwardly (as viewed in FIGURE 7) from the glassseal 54. This can be accomplished by using an etchant to remove aportion 66 of the tube down to, e.g., the dash-dot line 64 in FIGURE 7.Removal of a substanti-al portion 66 of tube 52 eliminates the danger ofa short circuit between uninsulated rod portion 67 and tube 52. Asubstantial portion 68 of glass seal 54 is not enclosed by tube 52 inthe embodiment wherein a portion 66 of tube 52 is removed by etching.

Referring now to FIGURE 9, there is illustrated the assembly offeedthrough embodiment 50 to a plate typically composed of stainlesssteel and having an opening 61 defined by a raised portion or shoulder62 upon which rests a flange 53 projecting radially outwardly at an endof tube 52. Tube 52 and shoulder 62 are connected together by welding orbrazing, at 63. The composition of tube 52, when the latter is composedof Kovar alloy, is as follows:

Element: Weight percent Nickel 29. Cobalt 17. Iron Remainder. Manganese0.50 maximum. Silicon 0.20 maximum. Carbon 0.06 maximum. Aluminum 0.10maximum. Magnesium 0.10 maximum. Zirconium 0.10 maximum. Titanium 0.10maximum.

The total of aluminum, magnesium, zirconium and titanium should notexceed 0.20 weight percent.

To prevent gas leakage through the end of rod 51 exposed to higherpressure Zone, the tip of rod 51 should 'be coated with brazing such asthe brazing 39 at the tip of rod 21 in FIGURE 2. Also, to prevent gasleakage through tube 52, it is important that the tube be composed of amaterial other than molybdenum, which in tubular form does not havesatisfactory gas 1eakage-pre venting properties.

Inasmuch as feedthrough 50 is assembled to a stainless steel plate 60(FIGURE 9) it would be desirable to utilize a tube 52 composed ofstainless steel, because this would facilitate the thermal attachment oftube 52 to plate 60. However, a stainless steel tube presents problemsin situations where gas leakage from the higher pressure zone throughthe feedthrough is undesirable. This is because the thermal expansionproperties of stainless steel are not substantially identical to thoseof molybdenum rod 51 and glass seal 54 (composed of glass of the typedescribed previously with respect to the composition of glass seal 27 inembodiment 20 illustrated in FIGURES 1 and 2). Thus, during heat-ups andcool-downs, such as those occurring during assembly and bake-outs of thefeedthr-ough, minute leakage openings could form between the glass seal54 and the tube 52, if the latter were composed of stainless steel.

However, in some situations where leakages of this nature are not aproblem (e.g., in space vehicles wherein the feedthrough extendedbetween a terminal on the interior of a space vehicle and a terminal inspace) stainless steel could be used for tube 55. In outer space thereis no disadvantage to have a minute leak-through of gases from theinterior of the space vehicle into outer space because the vacuum inspace would not be effected. An advantage of a stainless steel tube isthat it may readily be brazed to a stainless steel plate 60 utilizing,e.g., a copper-gold brazing alloy. This is much simpler than thehell-arc welding procedure required to assemble a Kovar tube to astainless steel flange.

An advantage of embodiment 50 is that a substantial portion of tube 52adjacent flange 53 is not filled with glass seal 54. Therefore, insituations where the tube is welded or brazed to plate 60, the glassseal is so far removed from the area where the welding or brazing isbeing performed, that the Welding and/or brazing has no undesirablethermal effect on the glass seal.

Referring now to FIGURE 10, there is illustrated a setup for assemblinga fourth embodiment 70 of a feed through constructed in accordance withthe present invention. The setup includes a graphite jig 73 which mountsa molybdenum wire or rod 71 in an upright position. At the top of rod 71is a molybdenum head 79. A glass mounting plate 74, having a compositionsimilar to that of the glass slugs 29 and 57 in embodiments 20 and 50respectively, rests atop jig 73 and includes an opening 75 through whichrod 71 extends.

The entire setup illustrated in FIGURE is subjected to heat, whereuponthe glass plate 74 softens allowing molybdenum head 79 to penetrate orpress downwardly into the softened glass 74. This effects a seal betweenhead 79 and glass 74. The resulting configuration is illustrated as 70in FIGURE 13.

After the components 71, 79 and 74 are assembled in the manner shown inFIGURE 13, the glass plate 74 is mounted so that the outer edges thereofrest atop a molybdenum tube 76 outwardly from the bottom of whichextends a flange 77 (e.g., of stainless steel) attached to tube 76 bybrazing as at 78. The setup illustrated in FIG- ure 11 is then subjectedto heat which causes glass 74 to soften and to sink downwardly relativeto tube 76, in effect causing the upper portions of tube 76 to penetrateor press into glass 74 and effect a connection between glass 74 and tube76. Because glass 74 and molybdenum tube 76 have similar thermalexpansion characteristics, subsequent heat-ups and/ or cool-downs willnot affect the leakproof seal between plate 74 and tube 76.

Referring to FIGURE 14, there is illustrated a setup for producing stillanother embodiment of a feedthrough constructed in accordance with thepresent invention. In the setup of FIGURE 14, a molybdenum rod or wire81 is supported in an upright position (e.g., by a jig not shown .butsimilar to that illustrated in the setup of FIGURE 10) and rod 81terminates at an upper molybdenum head 89 connected to rod 81. Head 89rests atop a glass slug 82 typically of the same glass composition asslugs 29 and 57 and including an opening 83 through which rod 81extends. Glass slug 82 is supported atop a sapphire mounting plate 84including an opening 85 through which rod 81 extends.

When the components are arranged in the setup illustrated in FIGURE 14,the entire setup is subjected to heat which causes the glass slug 82 tosoften, and the weight of molybdenum head 89 causes the latter to pressdownwardly into glass slug 82, to effect a seal between head 89 and theglass slug. At the same time, the softened glass slug flowssubstantially into the configuration illustrated in FIGURE 15, and, inaddition, forms a tight, leak-proof bond with sapphire plate 84. Theresulting embodiment is illustrated generally at 80 in FIGURE 15 andincludes a molybdenum wire or rod 81 terminating at a molybdenum head89, sapphire plate 84, and a glass seal 86 binding the components 81, 89and 84 together into a leak-proof, gas-impervious feedthrough.

In embodiment 80 there is a minimum amount of glass as a component ofthe feedthrough, compared, for example, to the embodiment 70 illustratedin FIGURE 11. Accordingly, the possibility of a gas leakage throughglass- 86 is further minimized. Sapphire plate 84 is not susceptible togas leakage, and the sapphire plate and glass seal 86 effect anextremely good mutual bond.

Referring to FIGURES 16 and 17, there is illustrated an assemblynormally mounting a pluraiity of feedthroughs such as 20. Assembly 140includes a plate 141, composed of molybdenum, and having a depending rim142 and a plurality of openings 143 each peripherally surrounded by arecessed plate portion 144. Flange 23 on cup 22 of feedthrough 20 has adownwardly depending terminal end portion which is received within andrests upon recessed plate portion 144; and feedthrough rod 21 extendsthrough opening 143. Flange 23 is secured to plate 141 at 145, e.g., bybrazing.

Because cup 22 and plate 141 are both composed of molybdenum, and thushave the same thermal expansion characteristics, there is no danger of aleak developing between the two during the expansion and contractionthereof caused by bakeout or other heating to which the assembly issubjected.

As previously described molybdenum rod 21 and cup 22 are chrome platedand the chrome is oxidized. Molybdenum plate 141 is subjected to thesame treatment. The resulting coating of chrome oxide on plate 141, oncup 22 and on rod 21 prevents the molybdenum, of which these threeelements are composed, from oxidizing at the high temperatures ofbakeout.

In brazing various components together, a brazing alloy which may beused advantageously in securing molybdenum to copper, molybdenum tomolybdenum, stainless steel to stainless steel, and copper to stainlesssteel, contains 65% copper and 35% gold with a 1010 C. liquidus. Tosecure Kovar to stainless steel, a brazing alloy of copper-gold-nickel,having 1030 C. liquidus, is advantageous.

A stainless steel which may be used for ring 34 and for tube 52 is AISItype No. 304 containing, in weight percent, 0.08 maximum carbon,18.0-20.0 chromium, 8.00 to 11.00 nickel, 2.00 maximum manganese, andthe balance iron, a non-magnetic, austenitic, readily weldable alloy.

In summary, the invention relates to a feedthrough for extension betweentwo zones of respectively differing pressures. The feedthrough, composedof materials which render it completely non-magnetic, includes anelongated, drawn, metallic, electricity-conducting first member havingopposite ends each for positioning in a respective one of the twopressure zones. Also included are a mounting member, having an openingthrough which the drawn member extends, and glass means constituting agas-leakpreventing seal between and in contact with the two members. Atleast the draw member and the glass means,

and preferably also the mounting member, have the same thermal expansioncharacteristics. Fused at that end of the drawn member which is to belocated in the zone of higher pressure is metallic sealing means forpreventing gas leakage through the end and between the elongated grainsin the miscrostructure of the drawn member.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. A gas-impervious, electrical feedthrough-for extension between twozones of respectively difiering pressures, said feedthrough comprising:

a drawn, elongated molybdenum first member for conducting electricitybetween said two zones;

said first member having a pair of opposite ends at least one of whichis a free end for positioning in a respective one of said two pressurezones;

said first member having elongated microcrystalline grains in itsmicrostructure;

a gas-impervious, metallic mounting member having an opening throughwhich said first member extends;

glass sealing means at said opening, between and in contact with saidfirst member and said mounting member, for preventing a gas leak throughsaid opening and for providing electrical insulation between the firstmember and the mounting member;

a tenacious oxide on at least those portions of said first member andsaid mounting member which are in contact with said glass sealing means;

said first member, said mounting member and said glass sealing meanshaving the same thermal expansion characteristics;

and metallic sealing means fused at said free end of said first memberfor preventing gas leakage longitudinally through the first member andthrough said free end;

said feedthrough being composed entirely of non-magnetic material.

2. A feedthrough as recited in claim 1 wherein:

said mounting member is composed of cross-rolled molybdenum sheet;

said glass sealing means is composed of alumino silicate glass;

and said metallic sealing means is composed of coppergold alloy.

3. A feedthrough as recited in claim 1 wherein:

said mounting member has flange extending in a direction axially of saidfirst member;

said flange being concentric with said opening and spaced therefrom in aradial direction relative to the opening.

4. In combination with the feedthrough of claim 3:

a plate having an opening therein;

said plate including a rim concentrically surrounding said plate openingat the opening;

said rim extending integrally from said plate;

said rim being slidably received within said flange on said mountingmember of the feedthrough;

said first member of the feedthrough extending through said plateopening;

and means securing said flange to said plate.

5. A combination as recited in claim 4 wherein:

said mounting member of the feedthrough is composed of cross-rolledmolybdenum sheet;

said plate is composed of copper;

said glass sealing means of the feedthrough is composed of aluminosilicate glass;

and said securing means is a brazement of copper-gold alloy.

6. In combination with the feedthrough of claim 3:

a plate having an opening therein;

said plate including a recessed plate portion concentric with andsurrounding aid opening at the periphery of the opening;

said flange on the mounting member of the feedthrough having a terminalend portion received in and supported by said recessed plate portion;

means securing said flange to said plate;

said first member of the feedthrough extending through said plateopening;

said mounting member being composed of cross-rolled molybdenum sheet andsaid plate being composed of molybdenum.

7. A gas impervious, electrical feedthrough for extension between twozones of respectively differing pressures, said feedthrough comprising:

a drawn, elongated molybdenum first member for conducting electricitybetween said two zones;

said first member having a pair of opposite ends at least one of whichis a free end for positioning in a respective one of said two pressurezones;

said first member having elongated microcrystalline grains in itsmicrostructure;

a gas-impervious mounting member having an opening through which saidfirst member extends;

glass sealing means at said opening, between and in contact with saidfirst member and said mounting member, for preventing a gas leak throughsaid opena tenacious oxide on at least those portions of said firstmember which are in contact with said glass sealing means;

said first member, said mounting member and said glass sealing meanshaving the same thermal expansion characteristics;

and metallic sealing means fused at said free end of said first memberfor preventing gas leakage longitudinally through the first member andthrough said free end;

said feedthrough being composed entirely of non-magnetic material.

References Cited by the Examiner UNITED STATES PATENTS 1,857,203 5/1932Van Liempt 174-50.61 2,174,375 9/1939 Beggs 174-152 X 2,292,863 8/1942Beggs 174152 X 2,450,780 10/1948 Bucklen 174152 X 2,458,748 1/1949Stupakotf 174-152 2,479,872 8/1949 Seiden 17450.56 2,508,551 5/1950Sykes 174152 X 2,807,074 9/1957 Schroeder 29497 X 2,933,635 4/1960 Langet a1 313-331 2,944,102 7/1960 Tidd l74-l52 FOREIGN PATENTS 166,32112/1955 Australia.

624,489 6/ 1949 Great Britain.

699,492 11/ 1953 Great Britain.

818,202 8/1959 Great Britain.

OTHER REFERENCES Hampel, Rare Metals Handbook, published by ReinholdPublishing Corp., New York, 1954, page 284.

Harwood, The Metal Molybdenum, published by the American Society forMetals, Cleveland, Ohio, 1958 pp. 117-118.

JOHN F. BURNS, Primary Examiner.

JOHN P. WILDMAN, E. JAMES SAX, Examiners.

1. A GAS-IMPERVIOUS, ELECTRICAL FEEDTHROUGH FOR EXTENSION BETWEEN TWOZONES OF RESPECTIVELY DIFFERIG PRESSURES, SAID FEEDTHROUGH COMPRISING: ADRAWN, ELONGATED MOLYDBENUM FIRST MEMBER FOR CONDUCTING ELECTRICITYBETWEEN SADI TWO ZONES; SAID FIRST MEMBER HAVING A PAIR OF OPPOSITE ENDSAT LEAST ONE OF WHICH IS A FREE END FOR POSITIONING IN A RESPECTIVE ONEOF SAID TWO PRESSURE ZONES; SAID FIRST MEMBER HAVING ELONGATEDMICROCRYSTALLINE GRAINS IN ITS MICROSTRUCTURE; A GAS-IMPERVIOUS,METALLIC MOUNTING MEMBER HAVING AN OPENING THROUGH WHICH SAID FIRSTMEMBER EXTENDS; GLASS SEALING MEANS AT SAD OPENING, BETWEEN AND INCONTACT WITH SAIF FIRST MEMBER AND SAID MOUNTING MEMBER, FOR PREVENTINGA GAS LEAK THROUGH SAID OPENING AND FOR PROVIDING ELECTRICAL INSULATIONBETWEEN THE FIRST MEMBERT AND THE MOUNTING MEMBER; A TENACIOUS OXIDE ONAT LEAST THOSE PORTIONS OF SAID FIRST MEMBER AND SAID MOUNTING MEMBERWHICH ARE IN CONTACT WITH AD GLASS SEALING MEANS; SAID FIRST MEMBER,SAID MOUNT MEMBER AND SAID GLASS SEALING MEANS HAVING THE SAME THERMALEXPANSION CHARACTERISTICS; AND METALLIC SEALING MEANS FUSED AT SAID FREEEND OF SAID FIRST MEMBER FOR PREVENTING GAS LEAKAGE LONGITUDINALLYTHROUGH THE FIRST MEMBER AND THROUGH SAID FREE END; SAID FEEDTHROUGHBEING COMPOSED ENTIRELY ON NON-MAGNETIC MATERIAL.